A Multiple-size-group CFD Model for an Internal Loop Airlift Reactor with Bubble Recirculation Regimes

Airlift reactors are used in a wide range of industries, such as hydrometallurgy, biochemical processes, chemical process industry and wastewater treatment. Despite the simple structure of airlift reactors, the flow field becomes complex with increasing gas velocity, and gas bubbles in the circulating regime can be observed in practice. In this paper, a numerical modelling method based on computational fluid dynamics (CFD) is presented for gas-liquid flow in airlift reactors under different bubble recirculation regimes. Gas-liquid flow was modelled using the Eulerian two-fluid equations, and extra user defined subroutines were incorporated to consider the complex physics, such as bubble-induced turbulence and turbulent dispersion force. Some alternative correlations for drag coefficient were tested to compare their ability to capture the bubble distributions in the riser and downcomer of the airlift reactors, with consideration of the interaction between bubbles. A model including multiple bubble sizes was applied to obtain more accurate simulation results of gas holdup and water velocity. Also, the use of the inhomogeneous multiple-size-group (MUSIG) model was explored as a way to better predict the complex flow regimes. The modelling method was applied to a laboratory internal loop airlift reactor, and the simulation results were compared with the published experimental measurements for gas holdup and water velocity. Reasonable agreement was obtained over a range of operating conditions, and an improvement was demonstrated using the proposed method. The simulations have shown that the inhomogeneous MUSIG model is a suitable tool to describe the complex gas-liquid interaction in the airlift reactor at a high gas superficial velocity.